1. Field of the invention
The present invention relates to a power protection apparatus and an electronic control unit including a switch unit serially connected to a power supply line extending from a power source to a DC regulator, a first short-circuit detector for detecting a short circuit based on an output voltage value of the DC regulator, obtained by making the switch unit conductive for predetermined time and, after that, interrupting the switch unit, and a resetting unit for resetting operation of the first short-circuit detector when the output voltage of the power source drops to a predetermined voltage.
2. Description of the related art
As shown in FIG. 1, in a DC regulator 100 constructed by a booster DC/DC converter, a MOS-FET 120 is controlled to be switched on or off by a booster DC/DC converter control IC 110, thereby adding energy accumulated in a coil 130 to energy of a power source 200 and outputting the resultant energy to a load circuit 300.
When something abnormal such as a short circuit occurs in the DC regulator 100 or the load circuit 300 in such a circuit configuration, unexpected high current flows from the power source 200 to the DC regulator 100 and the load circuit 300. There is consequently the possibility of a failure in an MOS-FET as a component of a switch circuit 400 for switching whether power is supplied from the power source 200 to the load circuit 300, a diode 140, the coil 130, and the like.
Consequently, as shown by a broken line in FIG. 1, by providing a switch circuit change-over unit 600 for turning on/off the switch circuit 400 and a microcomputer 500 for controlling the switch circuit change-over unit 600, the microcomputer 500 executes a primary check for determining whether an abnormality such as a short circuit occurs in the DC regulator 100 or the load circuit 300 or not at the start of power supply to the load circuit 300. When it is determined that an abnormality occurs, the switch circuit 400 is turned off via the switch circuit change-over unit 600, thereby preventing unexpected high current from being passed to a device which may fail.
The primary check will be described in detail below. As shown in FIG. 2, the microcomputer 500 turns on the switch circuit 400 for a predetermined period (30 msec in FIG. 2) via the switch circuit change-over unit 600, after that, turns off the switch circuit 400, and monitors a transient characteristic of output voltage of the DC regulator 100.
Predetermined charges are accumulated for the predetermined period in a capacitor 150 provided on the output side of the DC regulator 100. After that, the switch circuit 400 is turned off. Consequently, the charges accumulated in the capacitor 150 are discharged.
In the case where an abnormality such as a short circuit occurs, the charges accumulated in the capacitor 150 are discharged more sharply than discharge in a normal state. Therefore, whether an abnormality such as a short circuit occurs or not can be determined based on whether a drop in the output voltage is sharp or not.
The microcomputer 500 determines that when the output voltage of the DC regulator 100 is lower than a preset threshold voltage at a predetermined timing after turn-off of the switch circuit 400, an abnormality such as a short circuit occurs.
In the primary check, however, when resistance in a portion where a short circuit occurs in the DC regulator 100 and the load circuit 300 is zero or has a small resistance value such as 100 mΩ, at the moment of turn-on of the switch circuit 400, over current flows in the DC regulator 100 and the load circuit 300. Consequently, the output voltage of the power source 200 decreases and the following problem occurs.
Usually, to prevent an erroneous operation or the like of the microcomputer 500, a resetting unit 700 for forcedly resetting the microcomputer 500 when the output voltage of the power source 200 becomes equal to or lower than the preset predetermined value is provided.
As described above, when the output voltage of the power source 200 drops to a predetermined value or less, the resetting unit 700 operates to reset the microcomputer 500. Consequently, the switch circuit 400 which has been on via the switch circuit change-over unit 600 for the primary check is turned off.
As a result, the output voltage of the power source 200 is restored. The primary check is executed again by the microcomputer 500 operating after the reset, and the switch circuit 400 is turned on. As long as the short-circuit state continues, the resetting unit 700 starts again by short-circuit current.
That is, there is a problem such that, the switch circuit 400 repeats turn on/off and, by over current flowing during the turn on/off, the devices such as the switch circuit 400, the diode 140, and the coil 130 may be destroyed.
To solve such a problem, as shown in FIG. 3, a configuration may be employed in which a resistor R100 for detecting whether over current flows to the load side or not is connected in series in the output line of the power source 200 and a voltage across the resistor R100 is detected by the microcomputer 500.
When the microcomputer 500 determines that over current flows to the load side based on the voltage across the resistor R100, the switch circuit 400 is forcedly turned off regardless of the result of the primary check.
However, to detect over current caused by a short circuit properly and early based on the voltage across the resistor R100, it is necessary to perform sampling at high speed. There is a problem such that the expensive, high-performance microcomputer 500 is necessary.
As another example of a circuit for detecting an abnormality of this kind, Japanese Unexamined Patent Application Publication No. 2005-121433 discloses a disconnection detecting circuit for detecting an abnormality such as a disconnection of a load circuit operating under PWM control.
In the disconnection detecting circuit, current flowing in a load circuit is detected by a current detecting circuit, and a result of comparison between a voltage corresponding to the detected current and a reference voltage is input to a D-type flip flop. APWM signal is input to a clock input terminal of the D-type flip flop, and presence or absence of disconnection is determined based on the comparison result input to the D-type flip flop around the falling timing of the PWM signal.